BRIDGES. 



71 



The easterly abutment and the six easterly 

 PUTS rest upon pile foundations. The western 

 abutment, nnd all tho other piers, rest upon 

 Hi>li,l ruck. The eastern abutment was built 

 within an old embankment of earth where the 

 water stood at about the level of the founda- 

 tion piks ; nnd tho abutment on the western 

 shore was built in water seventeen feet deep. 

 Tho di-pth of water at tho several piers is as 

 follows : .it pier one, 21 feet 2 inches, and suc- 

 cessively 19 foot 2 inches, 88 feet 5 inches, 7 

 5 inches. 9 feet 10 inches, 31 feet C inches, 

 80 feet 8 inches, 81 feet 4 inches, 25 feet, 22 

 17 feet G inches, and 11 feet. 



Coffer-dams could not have been used upon 

 the foundations of this bridge with any chance 

 of success, except where the water is shallow, 

 or rather where it is of ordinary depth, for it is 

 nowhere of* much less depth than the St. Law- 

 rence, where it is the deepest at the site of the 

 Victoria Bridge; nor could pneumatic piles 

 have been used here but in exceptional cases. 

 It would have been hazardous in the extreme 

 also to have attempted to use tho method 

 adopted by Mr. Brunei at the Salrash Bridge. 

 The ruder and more unscientific methods, de- 

 pendent more or less upon chance for their effi- 

 cacy, which are sometimes resorted to by engi- 

 neers in difficult situations, were altogether 

 unavailable here, for various reasons. The 

 means actually employed, therefore, for effect- 

 ing the nnder-water work were necessarily 

 somewhat different from the ordinary, and 

 consisted mainly in the use of portable iron 

 caissons sunk upon prepared foundations, partly 

 by tho use of screws, and partly by means of 

 guide piles only. Where the foundations were 

 of piles, these were driven as far as was possible 

 with a ram weighing 2,200 pounds, and wore 

 sawed off at a level as much below the river 

 bod as was practicable. The sawing was ef- 

 fected by a very simple machine, which, accu- 

 rately did its work in depths of water exceed- 

 ing forty-two feet, at the rate sometimes of 



sixty piles per day. At one of the pier* vrbert 

 t he \\uter was thirty-nine feet above the founda- 

 tion piles, a construction wharf was built around 

 the site in the manner shown in the accompa- 

 nying drawing. The caisson of thU pier wat 



MODE OF 



PIERS AT SU8QUEIIANNA BRIDGE. 



' ' i . i rr i 



1 



_LB 



nc.z. 



fastened to a timber platform, four feet thick. 

 The platform was made to move vertically 

 within guides attached to these constructed 

 wharves. Three arms projected from each side 

 of tho platform. Screws of three and a half 

 inches in diameter and fifty-six feet long, se- 

 cured to simple turning-gear erected upon the 

 deck of tho wharves, were passed vertically 

 through nuts contained in these arms. Upon 

 the screws turning horizontally, and having no 

 other movements, tho pier was made to de- 

 scend, or, if required by any exigency, to move 

 in the opposite direction. This movement is 

 excellently well illustrated by the elevators 

 used at hotels. Tho caisson was designed to 

 be water-tight. The boiler-plate iron used was 

 three-eighths of an inch thick from the bottom 

 to within ten fee* of tho surface of the water, 

 and elsewhere one-quarter of an inch thick. 

 It was made rigid by angle iron at- 

 tached to the sides and ends in rows 

 about seven feet apart. During the 

 process of lowering, it was heavily 

 braced inside with oak timber, to 

 strengthen it against the pressure 

 of tho water outside, which at some 

 points in the descent was sixteen 

 pounds to the square inch. 



The superstructure of this bridge 

 has some peculiarities. It was ori- 

 ginally designed to be of iron, but 

 when tho time came for its erec- 

 tion that material could not be 

 procured of the requisite quality 

 with that promptness which the 

 emergency required, and, though 

 with great reluctance on the part 

 of the engineer, timber was em- 

 ployed as a substitute. The chorda 

 o' the trusses vary in their dimen- 



